Update fast_line_detector.cpp

Author: sturkmen72

modules/ximgproc/src/fast_line_detector.cpp

@@ -108,16 +108,17 @@ void FastLineDetectorImpl::drawSegments(InputOutputArray image, InputArray lines
 {
     CV_INSTRUMENT_REGION();
 
-    CV_Assert(!image.empty() && (image.channels() == 1 || image.channels() == 3 || image.channels() == 4));
+    int cn = image.channels();
+    CV_Assert(!image.empty() && ( cn == 1 || cn == 3 || cn == 4));
 
-    if (image.channels() == 1)
+    if (cn == 1)
     {
         cvtColor(image, image, COLOR_GRAY2BGR);
     }
     else
     {
         cvtColor(image, image, COLOR_BGRA2GRAY);
-        cvtColor(image, image, COLOR_GRAY2BGR);
+        cvtColor(image, image, cn = 3 ? COLOR_GRAY2BGR : COLOR_GRAY2BGRA);
     }
 
     double gap = 10.0;